1. Field of the Disclosure
The present disclosure generally relates to electronic devices and, more particularly, to a system and method to detect row-to-row shorts and other row decode defects in semiconductor memory chips.
2. Brief Description of Related Art
Memory devices are electronic devices that are widely used in many electronic products and computers to store data. A memory device is a semiconductor electronic device that includes a number of memory cells, each cell storing one bit of data. The data stored in the memory cells can be read during a read operation. FIG. 1 is a simplified block diagram showing a memory chip or memory device 12. The memory chip 12 may be part of a DIMM (dual in-line memory module) or a PCB (printed circuit board) containing many such memory chips (not shown in FIG. 1). The memory chip 12 may include a plurality of pins 24 located outside of chip 12 for electrically connecting the chip 12 to other system devices. Some of those pins 24 may constitute memory address pins or address bus 17, data (DQ) pins or data bus 18, and control pins or control bus 19. It is evident that each of the reference numerals 17-19 designates more than one pin in the corresponding bus. Further, it is understood that the schematic in FIG. 1 is for illustration only. That is, the pin arrangement or configuration in a typical memory chip may not be in the form shown in FIG. 1.
A processor or memory controller (not shown) may communicate with the chip 12 and perform memory read/write operations. The processor and the memory chip 12 may communicate using address signals on the address lines or address bus 17, data signals on the data lines or data bus 18, and control signals (e.g., a row address select (RAS) signal, a column address select (CAS) signal, etc. (not shown)) on the control lines or control bus 19. The “width” (i.e., number of pins) of address, data and control buses may differ from one memory configuration to another.
Those of ordinary skill in the art will readily recognize that memory chip 12 of FIG. 1 is simplified to illustrate one embodiment of a memory chip and is not intended to be a detailed illustration of all of the features of a typical memory chip. Numerous peripheral devices or circuits may be typically provided along with the memory chip 12 for writing data to and reading data from the memory cells 26. However, these peripheral devices or circuits are not shown in FIG. 1 for the sake of clarity.
The memory chip 12 may include a plurality of memory cells 26 generally arranged in rows and columns to store data in rows and columns. A row decode circuit or row decoder 28 and a column decode circuit or column decoder 30 may select the rows and columns in the memory cells 26 in response to decoding an address provided on the address bus 17. Data to/from the memory cells 26 is then transferred over the data bus 18 via sense amplifiers and a data output path (not shown in FIG. 1, but shown in FIG. 2). A memory controller (not shown) may provide relevant control signals (not shown) on the control bus 19 to control data communication to and from the memory chip 12 via an I/O (input/output) circuit 32. The I/O circuit 32 may include a number of data output buffers or output drivers to receive the data bits from the memory cells 26 and provide those data bits or data signals to the corresponding data lines in the data bus 18.
The memory controller (not shown) may determine the modes of operation of memory chip 12. Some examples of the input signals or control signals (not shown in FIG. 1) on the control bus 19 include an External Clock signal, a Chip Select signal, a Row Access Strobe signal, a Column Access Strobe signal, a Write Enable signal, etc. The memory chip 12 communicates to other devices connected thereto via the pins 24 on the chip 12. These pins, as mentioned before, may be connected to appropriate address, data and control lines to carry out data transfer (i.e., data transmission and reception) operations.
A test mode control unit 34 is also illustrated as part of the memory chip 12. The test mode control unit 34 may include digital logic such as, for example, one or more test mode registers to perform testing of the memory chip 12 for example, during and after fabrication of the chip 12, as discussed later. A memory controller (not shown) may instruct the control unit 34 to generate and send appropriate test-related signals to the chip 12 during the test phase.
FIG. 2 is a simplified architecture for the memory device 12 shown in FIG. 1. It is evident that complex circuit details and constituent architectural blocks in the memory chip 12 are omitted from FIG. 2 for the sake of clarity and ease of illustration. As shown in FIG. 2, a data storage or memory array consists of a matrix of storage bits or memory cells 26, each bit being exclusively referenced by a corresponding row and column address (that may be present on the address bus 17). In the example of FIG. 2, the memory array consists of 2m×2n bits. Each row of memory cells may be called a “wordline” 36, whereas each column of memory cells may be called a “digitline” 35. In FIG. 2, there are 2m rows addressable by the “m” row address lines input to the row decoder 38. Similarly, there are 2n columns addressable by the “n” column address lines input to the column decoder 30. However, for ease of illustration, only one row (wordline 36) is shown in FIG. 2, and a few digitlines 35 are partially shown. Each memory cell or bit 26 may have a unique column address and row address associated with it as can be seen from the physical placement of memory cells 26 illustrated in FIG. 2. That is, each memory cell 26 may be connected to only one digitline 35 and only one wordline 36. A memory cell 26 may include a 1-transistor 1-capacitor (1T1C) design as is known in the art.
During a memory “activate” command, a row address is read in (from the address signals on the address bus 17 as is known in the art) and the row decoder 28 selects one of the 2m rows or wordlines 36 depending on the combination of “m” bits present in the received row address. All 2n cells 26 along this selected wordline 36 are activated and the data that is stored on each cell is routed to a sense amplifier 38 via digitlines 35. The sense amplifier 38 magnifies each bit of data that is stored to an appropriate voltage level (e.g., a “low” voltage level to represent a binary digit “0” and a “high” voltage level to represent a binary digit “1”) at each activated cell 26. Next, the column decoder 30 selects one bit 26 out of the 2n activated bits as is shown by the darkened bit 26 along a filly-drawn digitline 35 in FIG. 2. The bit chosen by the column decoder 30 is routed from the sense amplifier 38 out of the memory cell array to other amplification circuitry and output buffer 40 (which may be part of the I/O circuit 32), which sends the addressed data bit out to the data requestor (e.g., a microprocessor or a memory controller (not shown)) over appropriate data line 18. Similarly, other memory cells may be read for their data content. A data write operation may be performed in a similar manner using appropriate data write circuitry (not shown) and, hence, is not described herein for the sake of brevity.
In modern memory designs, each wordline 36 may be connected to a negative wordline voltage (VNWL) (not shown in FIG. 2, but illustrated in FIG. 4) to reduce leakage current when the corresponding wordline is “off” or “inactive” as is known in the art. It is observed here that row-to-row (i.e., wordline-to-wordline) shorts have always existed on memory devices such as, for example, DRAM (Dynamic Random Access Memory) chips, upon fabrication. However, with the connection of wordlines to the negative wordline voltage (VNWL), it may be possible that shorted rows end up disturbing VNWL and, hence, increasing leakage current and associated power consumption. Therefore, it is desirable to devise a mechanism to detect wordline shorts in memory devices (or similar shorts in other electronic devices), while limiting the current supplied to the row decoder associated with the shorted wordlines. It is further desirable that the devised mechanism be useful in curing other row decode defects such as, for example, preventing overstress during burn-in testing of a memory device (or an electronic device) to remove infant failures.